Minerals I
Elements of the Earth’s crust
Properties of minerals
Crystal form
Cleavage/Fracture
Color, streak, luster
Hardness, tenacity
Specific gravity
Acid test, magnetism, etc.
http://www.nps.gov/seki/snrm/geology/cave_photos.htm
Why Study Minerals?
Building blocks of Earth materials
Economically important
Essential to understanding rock formation
Minerals are cool
Mineral - A naturally occurring, inorganic,
homogeneous solid with a definite chemical composition
and an ordered atomic arrangement.
naturally occurring - materials synthesized in laboratory do not count,
must be formed by natural processes in wild
inorganic - not formed by organic chemistry (e.g., sugars, etc.)
homogeneous solid - single substance in solid phase which cannot
be physically separated into simpler compounds
definite chemical composition - composition can be represented as
chemical formula (e.g., NaCl), although variation in exact composition
is possible (e.g., (Mg, Fe)2SiO4 )
ordered atomic arrangement – repeating structure at the atomic level,
which expresses as symmetry in large specimens, i.e., crystalline
structure.
http://www.uoregon.edu/~jrice/geol311
Mineral Formation
Minerals found at the Earth’s surface formed from natural
processes both within the Earth and at the Earth’s
surface. Environments vary greatly, and include:
High temperature and pressure typical of the upper mantle.
Evaporation basins
Ocean bottoms
Volcanic calderas and lava flows
A host of stressful environments in mountain building zones
And thousands of others!
Mineral Properties
Crystal Form
Cleavage/Fracture
Color
Streak Color
Luster
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Most useful properties for mineral identification
Mineral Properties
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Crystal Form
Cleavage/Fracture
Color
Streak Color
Luster
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Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Crystal symmetry is controlled by the
atomic structure of mineral
90o
4-fold symmetry (90o)
Symmetry
4-fold axes (90o)
http://www.a-m.de/englisch/lexikon/mineral/halogenide/halit-bild1.htm
http://home.planet.nl/~bartdw/sphalerite.htm
3-fold symmetry (120o)
Symmetry
3-fold axes (120o)
Crystal Form
There are only 6 symmetry classes; each mineral belongs to only ONE
symmetry class, and thus all crystals exhibit that symmetry only!
isometric
tetragonal
orthorhombic
hexagonal
monoclinic
triclinic
http://members.aol.com/jmichaelh/part1.html
Crystal Form
Caveat One: Crowding of
Caveat Two:
other crystals can mask
symmetry
Crystals are often too
small to see
Symmetry can always be
seen using techniques
like X-ray diffraction
Mineral Properties
• Crystal Form
Cleavage/Fracture
• Color
• Streak Color
• Luster
•
•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Cleavage - tendency to split along planes of weakness
corresponding to weaker chemical links in internal
structure of crystal
A single crystal can display several cleavage directions.
1 direction
2 directions
The cleavage directions are not always at right angles to each other. They
also are not necessarily parallel to the crystal faces of the mineral.
Parallel Cleavage
Cleavage
Can be parallel to
crystal faces, but is
often at an angle
One Direction
of Cleavage
Fig. 02.19a
One Direction
of Cleavage
Two Directions
of Cleavage
Fig. 02.21
Three Directions
of Cleavage
Fig. 02.22
http://www.geology.sdsu.edu/visualgeology
Four Directions of
Cleavage
Fluorite
Crystals
http://www.xpsdata.com/
http://www.greatsouth.net/
Mineral Properties
• Crystal Form
Cleavage/Fracture
• Color
• Streak Color
• Luster
•
•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Fracture - any breakage that is not cleavage
conchoidal
fracture
Characteristic of quartz,
chert and flint
conchoidal
fracture
Fig. 02.23
Mineral Properties
• Crystal Form
• Cleavage/Fracture
Color
• Streak Color
• Luster
•
•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Characteristic for some minerals, useless for most
One mineral can come in a rainbow of colors
Spinel
MgAl2O4
Hardness = 8
Quartz
Jasper
Citrine
Rose Quartz
Quartz
SiO2
Amethyst
Hardness = 7
Carnelian
Onyx
Smokey
Quartz
One mineral can appear to be different colors
under different lighting
Chrysoberyl
sunlight
BeAl2O4
Hardness = 8.5
Alexandrite
artificial
light
Mineral Properties
• Crystal Form
• Cleavage/Fracture
• Color
Streak Color
• Luster
•
•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Streak Color - Color of powdered mineral, which
may or may not be the color of the crystal
hematite streak color
Mineral Properties
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•
•
•
Crystal Form
Cleavage/Fracture
Color
Streak Color
Luster
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•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Luster - quality of light reflected from mineral surface
Luster - quality of light reflected from mineral surface
pyrite
metallic
gold
silver
Luster quality of light reflected from mineral surface
non-metallic
pearly
vitreous (glassy)
resinous
Mineral Properties
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•
•
•
•
Crystal Form
Cleavage/Fracture
Color
Streak Color
Luster
•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Hardness - resistance of mineral
to scratching or abrasion
Mohs Hardness Scale
Austrian mineralogist F. Mohs, 1824
hard
Higher numbered minerals
Based on relative
scratch lower numbered minerals hardness of 10
common minerals
10. Diamond C
9. Corundum Al2O3
8. Topaz Al2SiO4(F,OH)2
7. Quartz SiO2
6. Orthoclase KAlSi3O8
glass plate
5. Apatite Ca5(PO4)3(F,Cl,OH)
4. Fluorite CaF2
copper penny
3. Calcite CaCO3
fingernail
2. Gypsum CaSO4
soft 1. Talc Mg3Si4O10(OH)2
Muscovite
Hardness < 2
Can scratch with fingernail (H=2.5)
Fig. 02.14
Fluorite
Hardness =4
Fig. 02.15c
Can scratch copper penny (H=3.5),
but not glass plate (H=5.5)
Apatite
Hardness =5
Fig. 02.15b
Can scratch copper penny (H<3.5),
but not glass plate (H=5.5)
Quartz
Hardness =7
Fig. 02.15a
Scratches glass plate (H=5.5).
Hardest of common minerals
Mineral Properties
•
•
•
•
•
Crystal Form
Cleavage/Fracture
Color
Streak Color
Luster
• Hardness
Tenacity
• Specific Gravity
• Acid Test
• Magnetism
Tenacity - resistance of the mineral to breaking
Mineral Properties
•
•
•
•
•
Crystal Form
Cleavage/Fracture
Color
Streak
Luster
• Hardness
• Tenacity
Specific Gravity
• Acid Test
• Magnetism
Specific Gravity - density of the material (g/cm3)
Specific Gravity
Related to both the atomic weight of the contained
elements, and how tightly the atoms are packed
Orthorhombic Carbonates
All minerals have same structure, only cation differs
Mineral
Formula
Cation
g/cm3
Aragonite
CaCO3
20
2.95
Strontianite
SrCO3
38
3.76
Witherite
BaCO3
56
4.29
Cerrusite
PbCO3
82
6.55
Specific Gravity
Related to both the atomic weight of the contained
elements, and how tightly the atoms are packed
Graphite
Diamond
http://geology.uvm.edu/vtlandforms/gl_graph.htm
Both are minerals composed of the element carbon
Ggraphite = 2.23 (loose)
Gdiamond = 3.51(tight)
Mineral Properties
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•
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Crystal Form
Cleavage/Fracture
Color
Streak
Luster
• Hardness
• Tenacity
• Specific Gravity
Acid Test
• Magnetism
Reaction to dilute acid; especially useful
in identifying carbonate minerals
Minerals like calcite react with weak acid, dissolving the
mineral and producing lots of bubbles (effervescence)
Mineral Properties
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•
•
•
•
Crystal Form
Cleavage/Fracture
Color
Streak
Luster
•
•
•
•
Hardness
Tenacity
Specific Gravity
Acid Test
Magnetism
Some minerals strongly attract magnets. Characteristic
of magnetite and some other minerals